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Why does my uv-vis spectrophotometer give negative absorbance?
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mickeylee
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In phosphorus determination I use a spec. The thermo evolution 160. It has a carousel and at the 6,7,1 and 2 positions only, negative values are obtained. I cleaned the cuvettes, redid blanks and did it over. Same results. This occurs only on some days, on others I get positive readings at the same positions. Help!

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A year ago

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A year ago

Provided you have ruled out the more obvious causes such as using exactly the same solns, balanced sample and blank beams prior to each run, and the phases of the moon, I think you have an intermittent electronic fault. Your spectrometer is probably not from this century and one of the circuits is failing intermittently in just some settings.  Does it happen on particularly hot days? Overheating can cause these problems.  If so, can cool the innards with a small fan (don't use the one in your computer).  But before you do that take it to the electronics shop and have them clean and check the connections.  It probably won't fail when they are checking it, however (one of the many corollaries of Murphy's Law).   They may have spare spectrometers that they can cannibalize the parts from and replace the defective board.  If that doesn't work you have to fly the technician in, but a new uv-vis spectrometer is probably cheaper (an IR or NMR spectrometer probably not).  You could also switch your spectrometer with your neighbors (or with one from the teaching lab).  Be sure to wipe your fingerprints off the defective one and immediately put your name on the "borrowed" one.
If you are a graduate remind your boss that even at the $1 an hour that he/she pays you, your time is still the most expensive item in the lab and YOU MUST HAVE EQUIPMENT THAT DOES NOT DRIVE YOU UP THE WALL!!!
And have fun knowing that people have had the same problem and survived. Smile



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